1. Field of the Invention
The present invention relates to a sheet-like light source device and a liquid crystal display device using the sheet-like light source device. More particularly, it relates to a sheet-like light source device which is employed as a backlight of a liquid crystal display device, which has high impact resistance and can be manufactured at a low cost even if the device is large in size, and which can suppress the occurrence of uneven brightness, and a liquid crystal display device using the surface light source device.
2. Discussion of the Background
In a liquid crystal display device, a backlight is employed to apply light onto a liquid crystal display element. As shown in FIGS. 4 to 5, a conventional backlight has a reflecting plate 4 arranged to face a light reflecting surface 3b of a flat light-conducting plate 3 and an optical sheet 6 arranged on the light emission surface 3c opposed to the light reflecting surface 3b. A light source (lamp) 1 is provided along the light incidence surface 3a of the light-conducting plate 3, and a reflecting member (reflector) 2 is arranged around the light source 1 so as to allow light from the light source 1 to be efficiently incident on the light-conducting plate 3. In many cases, a fluorescent tube is employed as the light source 1. One or a plurality of fluorescent tubes are arranged relative to one side surface of the light-conducting plate 3. The light sources 1 are sometimes arranged on a plurality of sides of the light-conducting plate 3, respectively. The optical sheet 6 is, for example, a plurality of prism sheets arranged to converge light in the direction of the angle of visibility of the liquid crystal display element 7. As shown in FIG. 4, the light source 1, reflecting member 2, light-conducting plate 3 and reflecting member (reflector) 2 are held by a holding member (frame) 5. This holding member 5 also forms a gap between the liquid crystal display element 7 and the light-conducting plate 3 so as to prevent excessive pressure from being applied onto the optical sheet 6. With the conventional structure, an optical structure cannot be formed on the side surface of the light-conducting plate 3 facing the light source 1, and the position of the light-conducting plate 3 cannot be, therefore, fixed using the side surface facing the light source 1. Due to this fact, as shown in FIG. 5, a protrusion 3d or a recess is formed on a side surface which does not face the light source 1 to thereby hold the position of the light-conducting plate 3. A structure to be fitted into this protrusion 3d is formed on the holding member 5. By fitting the structure into the protrusion 3d, the light-conducting plate 3 is fixed.
However, due to formation of the protrusion 3d for fixing to the light-conducting plate 3, the following problems occur: (1) Force concentrates on the protrusion 3d when impact load is applied. If the surface light source device is larger in size, in particular, the weight of the light-conducting plate increases and the light-conducting plate cannot resist the impact load, thereby causing a crack. (2) Due to the optical structure provided on the side surface of the light-conducting plate 3, abnormal light emission derived from the optical structure occurs. (3) The formation of the protrusion 3d pushes up the unit price of the light-conducting plate or the cost of a die for forming it. (4) The light source 1 cannot be arranged on the side surface having the protrusion 3d. (5) If the light-conducting plate 3 has a prism function, abnormal light emission due to the protrusion 3d widely occurs on the surface.
The present invention has been made to solve the above-mentioned problems. It is an object of the present invention to provide a sheet-like light source device and a liquid crystal display device using the sheet-like light source device which can ensure high impact resistance even if the sheet-like light source device is large in size, which can be manufactured at a low cost and which can suppress the occurrence of uneven brightness.
In accordance with the present invention, there is provided a sheet-like light source device of side-light type comprising a light-conducting plate made of transparent material; at least one rod-like light source arranged to be adjacent to a light incidence surface which is at least one side of the light-conducting plate, a tubular reflecting member covering portions of the rod-like light source other than a surface facing the light-conducting plate, a planar reflecting member disposed proximate to a position facing a lower surface of the light-conducting plate, and a holding member housing the light-conducting plate, the light source and both of the reflecting members, wherein a stepped portion is provided on the holding member to contact a part of a side of the light-conducting plate on which the rod-like light source is arranged, with the holding member for holding the light-conducting plate, and a distance between the light-conducting plate and the holding member on the stepped portion is made narrower than a distance between an opening portion of the tubular reflecting member and the light-conducting plate.
Further, a liquid crystal display device according to the resent invention is characterized by disposing a liquid crystal display element on the sheet-like light source device.